1. Field of the Invention
This invention relates to a data receiving apparatus for receiving data and particularly to a data receiving apparatus used for a digital mobile unit communication with a phase correction.
2. Description of the Prior Art
A data receiving apparatus used for a digital mobile unit communication with a phase correction is known. In such data receiving apparatus, a phase shift keying (PSK) method is used. In the phase shift keying method, for example, data "1" is transmitted by advancing a phase of a carrier signal and data "-1" is transmitted by delaying the phase of the carrier signal. In the phase shift keying method, the carrier signal is given by: EQU E(t)=Ecos (.omega..sub.o t+.theta.(t))
where .omega..sub.o is a frequency of the carrier signal.
In the quadri phase shift keying method (QPSK), at least four kinds of data is transmitted through combination of data represented by phase shifting two carriers having a phase difference of .pi./2 each other.
If there is no phase difference between an oscillation frequency used for transmitting the carrier signal on the transmission side and the oscillation frequency used for demodulation on the receiving side, and in-phase component(real number component) Ri and quadrature component(imaginary number component) Rq are given by: ##EQU1## wherein each of the second terms are suppressed by a low-pass filters. Therefore, these equations are rewritten by: EQU Ri(t)=1/2.times.Ecos.theta.(t) EQU Rq(t)=1/2.times.Esin.theta.(t).
A data receiving apparatus employing this method judges that .theta.(t) is equal to 0, .pi./2, .pi., or-.pi./2 .pi., so that a symbol is decoded in accordance with the combinations of the in-phase component Ri(t) and the quadrature component Rq (t), namely, (E/2, 0), (0, E/2), (-E/2, 0) and (0, -E/2) are discriminated.
When there is a constant frequency difference .omega..sub.d between the transmission and receiving sides, the received signal is represented by: ##EQU2## wherein the second terms are suppressed by low-pass filters. Therefore, they are given by: EQU Ri(t)=1/2.times.Ecos(.theta.(t)-.omega..sub.d t) EQU Rq(t)=1/2.times.Esin(.theta.(t)-.omega..sub.d)t
As mentioned above, when there is a difference between the carrier frequency on the transmission side and a signal for demodulating on the receiving side, there is a phase difference represented by a linear expression in the received signal. Therefore, the data receiving apparatus is required to predict this linear expression and to correct the phase difference before discrimination of the received data.
FIG. 7 shows a graph of a phase error line used in the prior art data receiving apparatus. This linear expressing is represented by a slant line when phase changes (.theta.(t)-.omega..sub.d t) are plotted at the first quadrant from symbols sequentially received irrespective of quadrants. This line phase error line can be predicted by calculation of square means error from respective plotted points.
The phase correction corresponds to rotating respective plotted points by an angle corresponding to the slant angle of the phase error line toward X coordinate. After this phase correction, data represented by the plotted points are discriminated.
FIG. 8 is a block diagram of such a prior art data receiving apparatus. The prior art data receiving apparatus comprises an equalizing portion 132 for equalizing the received signal from an input terminal 131 to remove a distortion or the like, a memory 133 for storing the received signal from the equalizing portion 132, a phase characteristic predicting portion 134 for predicting the phase error line from a phase difference in the output signal of the equalizing portion 132, a correction portion 135 for correcting the phase difference in the output signal of the equalizing portion 132 using a rotation angle for correction determined by the slant angle of the phase error line, and a discrimination portion 136 for discriminating data represented by the phase corrected received signal to output a discrimination result from an output terminal 137.
FIG. 9 shows a flow chart representing an operation of this prior art data receiving apparatus.
In step s1, when the received signal is inputted to the input terminal 131, the equalizing portion 132 effects equalizing processing and outputs an equalized output signal. In the following step s2, the output of the equalizing portion 132 is stored in the memory 133. In the following step s3, the phase characteristic predicting portion 134 predicts a phase error line in accordance with the phase difference successively obtained in the output signal of the equalizing portion 132. In the following step s4, the phase characteristic predicting portion 134 supplies an inclination of the phase error line to the equalizing output correcting portion 135, the equalized output correcting portion 135 determines a correction angle for correcting the phase difference of the equalized output signal.
In the following step s5, the equalized output correction portion 135 corrects the phase of the equalized output signal read from the memory 133 by the correction angle, and supplies a corrected equalized output signal successively to the discrimination portion 136.
In step s6, the discrimination portion 136 discriminates the compensated equalized output into multi-values of the transmitted data and outputs the discriminating result from the output terminal 137.
As mentioned above, the prior art data receiving apparatus determines the phase error line representing phase errors from the equalized output signal, corrects the equalized output signal with the rotation angle for correction obtained by the inclination of the phase error line, and then, effects decoding.
However, there is a problem that it is very difficult to provided a high speed data processing in response to the data input because an amount of operation for presuming the phase difference between the transmission and receiving sides is extremely large.